Receive diversity is a simple technique which can be used to combat the fading effects due to multipath propagation in wireless communication systems. When two antennas are available at the receiver, information of the two radio channels can be exploited to provide increased reliability in data detection. Receive diversity techniques are well known and are widely adopted at the base station receiver (uplink) but not on the mobile equipment (downlink) since they have been considered too complex to implement in a low cost terminal. The complexity of receive diversity is due to the duplication of both the RF chain and base processing. In fact, to perform receive diversity is similar to having two User Equipments (UEs) in the same handset. Hence, a receive diversity with low complexity is recommendable for practical implementation in mobile terminals.
Proposals for reducing the cost and complexity of diversity receivers include U.S. Pat. No. 5,761,613 which discloses a receiver architecture comprising first and second reception branches coupled to respective antennas and including frequency down conversion means for frequency down converting respective radio frequency input signals to zero IF signals. The zero IF signals are digitised and combined in a maximal ratio combiner, the output from which is applied to a digital demodulator. In order to reduce the cost of the receiver the first and second branches are asymmetrical with say the first branch having a fully specified architecture and the second branch having a degraded architecture relative to the first branch by for example the omission of a first RF filter and an RF amplifier.
US Patent Publication No. US 2002/0126745 A1 discloses a receiver diversity architecture comprising a single antenna coupled to at least two Rake fingers. The Rake fingers are combined at chip level and then the combined signal is despread in a Walsh despreader to produce a symbol stream which is deinterleaved and decoded. The described diversity receiver is simpler and cheaper compared to an architecture in which each Rake finger includes its own despreader and produces time and phase corrected symbols which are combined, deinterleaved and decoded.
S. M. Alamouti “A Simple Transmit Diversity Technique for Wireless Communications” IEEE Journal on Select Areas in Communications, Vol. 16, No. 8, October 1998, Pages 1451 to 1458, discloses a simple two-branch transmit diversity scheme using two transmit antennas and one receive antenna. In the two transmit antenna/one receive antenna scheme disclosed in FIG. 2 of this article, two signals s0 and s1 are simultaneously transmitted a first symbol period from respective first and second antennas. In a second, consecutive symbol period the inverse complex conjugate of the signal s1, that is −s1*, is transmitted from the first antenna and the complex conjugate of the signal s0, that is s0*, is transmitted from the second antenna. The article also takes into consideration the additional matters of interference and noise and estimates of the respective signal channels and uses a maximum likelihood combiner to determine the values of s0 and s1. The disclosed scheme provides the same diversity order as maximal-ratio-receiver combining (MRRC) with one transmit antenna and two receive antennas. The described scheme does not require any bandwidth expansion and any feedback from the receiver to the transmitter, and its computation complexity is similar to MRRC.
The third generation mobile telephone system 3GPP, for example UMTS, requires the UE to have a Space Time Transmit Diversity (STTD) derotator which processes the signals received by two transmit antennas in two consecutive symbol periods and combines them to achieve a signal which has particular properties including an improved signal quality and thereby an improved receiver performance.
Reference is also made to unpublished PCT Patent Application IB 2007/051854 (Applicant's reference PH 005385EP1) which discloses a low cost and low complexity inner communication receiver which has an architecture enabling receiver diversity to be implemented using a STTD rotator. The receiver comprises first and second receive antennas for receiving first and second signals encoded with their respective channel coefficients. Each of the first and second signals comprises two consecutive signals occurring one symbol period apart. The first and second antennas are coupled by respective delay means to a master Rake receiver and a slave Rake receiver. For mobile receivers compliant with the 3GPP standard, the master Rake receiver is used in a classic estimation procedure which makes use of a pilot channel sequence (CPICH) for estimating the first and second channel coefficients. The slave Rake receiver is used to determine first and second auxiliary composite signals. The first and second auxiliary composite signals are combined with the estimates of the first and second channel coefficients to determine first and second output signals representative of the first and second symbols respectively times the sum of the first channel coefficient squared and the second channel coefficient squared. Although the described receiver enables the symbols to be recovered satisfactorily, a perceived drawback of the receiver architecture described is that the signals received from the first and second antennas in two consecutive time periods are summed doubling the noise power (3 dB loss), which loss is undesirable.